The invention relates to a magneto-optical recording apparatus for recording an information signal on a magneto-optical record carrier, which recording apparatus comprises a field coil for generating a magnetic field in the record carrier, an energizing circuit for energizing the field coil, and a control circuit for controlling the energizing circuit depending on the information signal in order to modulate the generated magnetic field in conformity with the information signal.
The invention further relates to an energizing circuit for use in such a magneto-optical recording apparatus.
For recording in magneto-optical materials it was customary until now to magnetise the magneto-optical material in a specific direction prior to recording and subsequently during recording to expose the material to a magnetic field whose direction is opposite to the direction of the previously applied magnetization and at the same time to hat the material locally by means of an intensity-modulated laser beam to a temperature near the Curie point, to produce a pattern of magnetic domains corresponding to the modulation pattern and having a direction of magnetization which differs from the surrounding area.
This method has the drawback that the previously recorded part must be erased before a new recording can be made.
A recording technique which mitigates this drawback is already known from Philips Technical Review, Vol. 42, no. 2, August 1985, page 41. In this recording technique the magnetic field is modulated instead of the laser beam. However, this recording technique using magnetic-field modulation is not utilised in practice because no satisfactory solution has been found for the circuit for energizing the field coil. The problem then encountered is that as a result of the strong magnetic field (approximately 200-300 oersted) the energy stored in the magnetic field is so high that the magnetic field can only be reversed rapidly enough by applying a large voltage difference across the magnetic coil, which in principle can be realised by means of voltage sources having a high output voltage (approximately 100 V). The field coil can then be energized via a series resistor to limit the steady-state current. In conventional magneto-optical recording apparatuses, in which the remainder of the electronic circuitry operates with a supply voltage of 5-15 V, magnetic-field modulation therefore means that an additional 100-V voltage source has to be used, which makes magneto-optical recording with a modulated magnetic field impracticable, the use of the series resistor has the disadvantage that a substantial amount of heat (approximately 10 W) is dissipated in the series resistor.